I was not refering to that chumrah. I was only answering that the fish would be considered fleishig and cannot be eating with milchig and also about Shabbos.
I would be ok with treating the fish as pareve if the pot hasn’t been used with meat in 24 hours.
See Pri Megadim S”D 95:2 regarding 3 NATs. His reasoning should apply here as well since there is no chashash that the fish touches the actual pot. Certainly if it is a stam kli there is reason to be meikel ( aino ben yomo -see Gr”a- possibly 3 NATs 2 Keilim, safek yad soledes etc.)
Regarding shabbos there are definitely reasons to be machmir but regarding shehiya/ hatmana I would look into whether the heating elements inside the sous vide unit actually come to a glow (red hot) or if they operate at a lower temperature which might not be considered aish.